1. Field of the Invention
The present invention relates to a coating apparatus and method for coating an object such as a semiconductor wafer with a coating liquid, such as, a resist, a developer or the like.
2. Description of the Related Art
In a pattern forming process, an electronic circuit pattern film is formed on a semiconductor wafer in the following manner. First, a resist film is coated on an undercoated thin film formed on the semiconductor wafer. Thereafter, the resist film is exposed through a mask having a predetermined pattern and developed, thereby forming a negative or positive resist pattern. Using the resist pattern as a mask, a desired electronic circuit pattern is formed on the undercoated thin film on the semiconductor wafer. Finally, the resist pattern used as a mask is removed.
If the thickness of the resist film coated on a semiconductor wafer is not uniform, the resist pattern cannot be formed accurately. Hence, it is necessary to form a resist film of uniform thickness. In particular, since the density of resist patterns has been increased in recent times, they are required to be more and more uniform in thickness.
In a resist coating step, a semiconductor wafer is generally subjected to an adhesion (AD) process at a temperature of about 90.degree. C., in order to increase the adhesion between itself and a resist layer. The processed wafer is cooled and then coated with a resist by means of a spin coater: i.e., a resist liquid is deposited on a central portion of the semiconductor wafer, which is then rotated at a high speed. As a result, the resist liquid spreads out toward the periphery of the wafer, and coats the overall surface thereof.
When a resist is coated on a semiconductor wafer by a spin coater, the thickness of the resist film varies in the central and peripheral portions of the semiconductor wafer, since the resist solvent is volatilized into the atmosphere at different rates in the central and peripheral portions of the wafer, due to the difference in their peripheral speeds. Since the diameter of wafers has become larger in recent years, difference in the thickness of resist films have tended to increase.
In a developing step, a developer is applied to the resist film on a semiconductor wafer which has been exposed, and the resist is developed by keeping the developer on the semiconductor wafer for certain time. Since the semiconductor wafer is rotated when or after the developer is supplied thereto and a gas flow in the apparatus is generated the developing rate is not uniform.